Electrical generation of musical tones



y 1950 M. J. LARSEN 2,506,723

ELECTRICAL GENERATION 0F MUSICAL TONES Filed Dec. 31, 1947 2Sheets-Sheet 1 I50 \6 H \3 t l2 l7 i FIG. I

Sme Wove Source E0 8 Audio Osci. B

FIG 2b FIG. 2a WES i V i E, Eb g Eb 0 Time O Time 50 5H 52 M L AudioFrequency E Amp.

\i 3 5 +B Audio Frequency Osoilioior INVEN TOR. Merwin J. Lursen AiivMay 9, 1950 M. J. LARSEN ELECTRICAL GENERATION OF MUSICAL TONES FiledDec. 51, 1947 2 Sheets-Sheet 2 H6 4 wEs H641 v a I E I 1 T/ (E0 V V T E0E .2' E E 0 .52 I: a:

Tlme- To 22; 74 "7 74 Audio 0- x 0 Amp. Sine Wave S e Wilve SourceSource 80 83 d Audio (Oscl Audio 78 Hor9no m'cs Audio 4 Amp 6| AllHormonlcs INVENTOR.

Merwin J. Larsen i atentecl May UNlTED sTATES mes-i arms Mei-win J.Larsen, Villa Park, 111., assignor to Stromberg-Carlson Company, acorporation of New York Application December 31, 1947, Serial No.795,060

8 Claims. 1

The present invention relates to apparatus for the electrical generationof musical tones and more particularly to improvements in musicalinstruments of the type employin electrical oscillators of audiblefrequency for the production of musical notes.

It is well-known that the timbre or tone quality of musical instrumentsdepends upon the harmonic content of the wave form of the musical noteproduced. Some musical instruments such as the flute, the triangle andthe like may produce tones having substantially sinusoidal wave forms.It is also well known that certain musical instruments are characterizedby the absence or extreme weakness of certain harmonics and thepredominance of others. Some musical instruments, for example, havestrong odd harmonics such as the fundamental, third, fifth, etc. andweak even harmonics such as the second, fourth, sixth, etc. Musicalinstruments of the type often characterized by the term electronicorgan, have been constructed so as to be able to imitate thecharacteristic of various other musical instruments having wave forms ofthe type referred to above. It is obvious therefore that such musicalinstruments must be able to produce tones varying from sinusoidal waveforms to those having complex wave forms. Known tone sources for use inelectrical musical instruments produce wave forms which are eithersubstantially sinusoidal or else Wave forms which are rich in harmonics.If a. sinusoidal Wave form source only is available, it is necessarysubsequently either to distort the sine wave or combine the outputs ofvarious sine Wave generators in order to arrive at a tone of complexwave form. On the other hand, if a complex wave form source only isused, such as relaxation oscillators, or multivibrators, then ratherinvolved filtering is necessary in order to produce tones having asubstantially sinusoidal wave form. g

It is apparent therefore that to produce the desired tone quality, it isnecessary to have tone sources which are capable of producing audiblesignals having wave forms varying all the way from asine wave to a waveform of very complex harmonic structure. A plurality of differentmethods have been employed for accomplishin this result. One methodwhich has been extensively employed comprises employing a plurality ofgenerators for producing only substantially sinusoidal voltages offrequencie following the tempered scale. The complex Wave form tones arethen produced by bleeding various harmonics along the tempered scalefrom those generators whose frequencies exactly or sometimesapproximately correspond to the harmonic orders desired and recombiningthem to produce the desired tones. Another method which has also beenextensively used employs means for initially generating tones rich inharmonics and employing suitable filters for getting the desired puretones Without har monies or the desired combinations of frequencies. Athird method starts with sinusoidal voltages and produces complex Waveforms by dis torting techniques. The present invention is partioularlyconcerned with this third method whereby a simple inexpensive multipletone source can readily be provided.

It is an object of the present invention to provide a, multiple tonesource for a musical instrument which is exceedingly simple andinexpensive and very satisfactory in operation.

It is another object of the present invention to provide a tone sourcewhich provides a source of substantially pure tones as well as a sourceof tones rich in harmonics without the requirement of filtering means orother complicated means for recombining various frequencies to providecomplex wave forms.

A still further object of the present invention is to provide a tonesource for both pure tones and tones having complex wave forms withsimple.

means for providing a vibrato output whenever,

desired.

Further objects and advantages of the present invention will becomeapparent as the following description proceeds, and the features ofnovelty; which characterize the invention will be pointed out withparticularity in the claims annexed to and forming a part of thisspecification.

For a better understanding of the present in-l' vention reference may behad to the accompanya ing drawings in which:

Fig. 1 is a schematic circuit diagram to aid in understandin one featureof the present invention;

Figs. 2a, and 2b are curve diagrams to aid in understanding theoperation of the circuit of Figs. 4a and 4b are curve diagrams similarto Figs. 2a and 2b to aid in understanding the operation of the multipletone source of Fig. 3;

Fig. 5 is a circuit diagram similar to Fig. 3

illustrating a modification thereof;

Fig. 6 is a curve diagram to aid in understanding the operation of themultiple tone source of Fig. 5; and

Fig. 7 is a diagram similar to Fig. 5 illustrat- 1 ing still anothermodification of the present in-- vention.

ence numeral H and the corresponding parts of Fig. 3 are designated bythe same reference numerals as in Fig. l. The audio frequency oscillatorII is illustrated as a parallel-T type oscillator which is a Specificone of the bridge type of resistance-capacitance oscillators. Asillustrated the oscillator H comprises a bridge circuit generallydesignated at 21 arranged in a so-called parallel-T with one Tcomprising the serially arranged resistors 22 and 23 and the capacitor24 connected between the common terminal of the resistors 22 and 23 andground. The other T circuit comprises the capacitors 25 and 26 connectedin series with the common terminal of the capacitors connected to groundthrough resistors '21 and 2-8, one of which, i. e. resistor 23, isindicated as being adjustable to provide means for obtaining a finefrequency adjustment as for initially tuning the electronic organ. Thisparallel- T bridge type circuit is disclosed in the Scott Patent2,173,427 referred to above. The audio frequency oscillator ll furtherincludes an electron discharge valve 30 specifically illustrated as atriode comprising an anode or plate 3!, a cathode 32 and a controlelectrode or grid 33. The cathode 32 is grounded through a suitablecathode resistor 34 which provides the desired degeneration forimprovement of the wave form, while the anode 3| is connected to asource of +B potential 35 through a suitable voltage dropping resistor36. The bridge circuit 2i described above has one terminal, namely theterminal between the resistor 23 and the capacitor 26, connected to thecontrol electrode 33 while the terminal between the resistor 22 and thecapacitor 25 is connected to the plate 35 through a suitable directcurrent blocking capacitor 38. A suitable resistor is also illustratedas normally connecting the control electrode 33 to ground as indicatedat 4|.

In order that a substantially pure sinusoidal potential may be obtainedfrom the audio frequency oscillator N there is provided an outputcircuit comprising the resistors 42 and 43 connected in series andconnected between ground and the plate 3! of the triode 30 preferablythrough the direct current blocking condenser 38 as illustrated. Thesinusoidal output voltage may be obtained at the terminal 44 between theresistors 42 and 43. The resistor 42 preferably provides a very highresistance so as to prevent any frequency change of the oscillator I!when an output potential is obtained as by keying or the like from theterminal 44.

Any suitable means for obtaining a sinusoidal potential from theterminal 44 may be provided. As illustrated the terminal 4% is connectedto a suitable movable switch arm 55 adapted to be actuated by a key 41pivotally mounted at 48. By depressing the finger engaging portion ofthe key 41 which may comprise the key of an electronic organ the switch45 is moved into engagement with the stationary contact 69 connected bya suitable conductor to an audio frequency amplifier 5| which in turnhas its output connected to a signal reproduction device or loud speaker52. It will be apparent therefore that the output of the audio frequencyoscillator I! may selectively be supplied to the amplifier 5| and loudspeaker 52 to produce an audible tone of substantially sinusoidalfrequency or in other words a pure tone. It will be understood that inan electronic organ a plurality of oscillators such as I I each capableof producing a difierent frequency will be provided and by suitablykeying these oscillators the common conductor 59 ma be selectivelyconnected to one or more of the oscilla various keying circuits of aplurality of oscillators such as l I.

It will be understood that other types of oscillators than the oneillustrated may be employed equally well herein, the only requirementsbeing that such an oscillator must have a safe margin of amplitude and ahigh degree of frequency stability.

By employing the parallel-T type oscillator described above a verydesirable arrangement is provided for the injection of a low frequencysignal to produce a frequency vibrato. As illustrated in the drawings asuitable key or stop 54 may be provided for controlling a two-positionswitch 55. Normally the switch 55 engages a contact 55a whereby thecontrol electrode 33 is grounded through the grid resistor 40. Uponactuation of the key or stop, however, the switch 55 is moved intoengagement with a normally open contact 55b so as to connect a lowimpedance low frequency source generally designated at 51 in series withthe resistor 43 with one terminal of the generator 51 connected toground, also designated by the reference numeral 4!. Preferably thegenerator 51 is a five to eight cycle generator of sufficiently lowimpedance whereby keying of the switch 55 has no adverse effect. By theproper choice of the resistance value of the resistor 40 the frequencyvibrato may be substantially free from amplitude modulation which isvery desirable since it permits the gas diode 12 to operate steadily. Itwill be apparent that a single generator such as 51 may be employed forsupplying the vibrato for all of the frequency oscillators such as l land consequently the conductor 60 connected to the other terminal of thegenerator 51 and the contact 55b may be common to all of the oscillatorssuch as H.

The gas diode or neon tube I2 is connected in substantially the samemanner as in Fig. 1 of the drawings with the resistor [3 connected tothe plate circuit of the triode 30. Instead of employing the biasingbattery of Fig. 1 of the drawings, however, the terminal |5a connectedto one electrode of the diode i2 is connected to the source of +3potential 35 through the serially arranged resistors 13 and. 36. Thesource of potential 35 therefore provides the necessary bias for the gasdiode l2 whose other electrode is connected to ground as shown. It willbe apparent that by suitably adjusting the resistors l3 and 36 anydesirable bias may be obtained such as was obtained by varying thevoltage of battery [4 in Fig.- 1 of the drawings.

The tone or signal of distorted wave form may also be keyed and, asillustrated in Fig. 3 of the drawings, the terminal 18a between theresistors l1 and I8 is connected to a suitable switch 6| which iscontrolled by a key 62 pivotally mounted at 63. The switch Bl includes anormally open contact 64 which is connected by a conductor 55' tosuitable filters generally designated at 66 the output of which may, ifdesired, be connected to the audio amplifier 5| for audible reproductionby the loud speaker 52. It will be understood that conductor 65 may be acommon circuit for the complex waves produced by the distorting cir-.cuits of other audio oscillators forming a part oi tially suppress thefundamental component/oi Resistor" ti; audit freuuency signatorcdmpi'exwate form sihcethisiundamental component is avanabierorseparatecontrolat theterminal M.- I

In Figs. 4"aand4b'there are illustrated certain waveforms-fonunderstandingthe operationof the distorting circuit of Fig. 3of the drawings-- which curves are q'uite similar to the curvesshown inFigs.- 2a -and 2b ofthe drawings. The curves E'il'both Figs.- 421"and-4b indicate the sinusoidal Voltage-output of the'audiooscillator II,it' beingobserved that a larger voltage is-p'resent'in the curve of Fig.4b which is obtained by increasing thepotential of the $13 source 35 1as by adjusting'theresistor 36: Thecurve's Ea of Figs; 4a and 1 46representthe potentialsat the terminal [512' oi tl ie 'dioder2 whichpotential is displaced from alsdbeing of-greater magnitude in Fig; 415

tHan=ih-'F*i .'-4aa v It'wiil 'be" understood thatvarious circuitarrangements and various circuit constants may be" employed 4 in"connection with" the ai -range: nie'nt ofFigz' 30f the drawings-forexample. In order, however; to illustrate the relative magnitildesof-the principal elements of a typical aircuilf arrangement which" hasbeen found to satis= factorily' embody the present invention the" fol-'lowing approximate values of suchelern'ents" to gether" with other"pertinent information" are gi iien foraparticular'devicei It shouldbeunder stoodtl iat these values are given byway of exaniple onlyaiidfno't by Way of limitation.

Gas'diodewa NE 9 /1; watt) Resistor-l3; 4:7 mego'hms Capacitor l6:has-such a value cf capac reactance at the 'oscii la-toi frequency is2.2

megohms; Resistor l1 .2 mego'h'ms" Resistor-l8" Gilmegohm' Resistor?0.27 megohin'" 0.27- megohm- Resistor 2e I r capacitance in' micrefar=capacitor 52 4;

adsapproximately equal" to 118/ wheref i's'the frequency; in cyclesCapacitor- 25: the capacitance 'o'fcapacitor "2e CEDRCWOFZE V2 thecapacitance of ca;-

paeitor' 24 18,000 ohms Oto l'OgOOO ohms Resistor 28' H Electrondischarge itan'cethat its capacitive" ssm of the csmt'c' be iiiritiorieu-"abeyef certaifi in'str-unfen notably the woodwindssuch as theclarinet, have strong odd harmctiicsand weak even fharmoni' Withthe'tcne source illustratedfinFiQ 3 of'th drawings it; is necessary: todo a: 'al' amount of filtering to eliminate the evenhe monies; InFig5"of" the drawings there is illusf trate'da niodification of thepresentinvention in which there is essentially provided am netorfesource whichcomprises a sine wavej tone' sourcea compleii wave tone sourcein whichsub? stantially only odd 'harmonics" are present; and a complex, waveform sourr'seinwhich 'substan ti'aliy all harmonics; are present.The'cor're' same reference numerals-as in the preceding figures. It willbe understood that the wave'Ee such as is shevvninFig. ib; fer example,could notbe use'dtoproduoe cl a net tone which containssubstantia'llyoniy odd harmonics, without "resorting to cumbersome filtering. ever,the Wave Ea of Fig. 4b, for example, were" distorted so both thepositiveand negative halves were'alike the resulting wave would contain sub- 5stantiallyonly odd harmonics; This is accomplished by the" circuitarrangement of' Fig. 5' where two gasl'diodesare' employed, designatedas lZ'aandiZb respectively; The diodes are conheated in series"with thec'oirinion terminal i0" 30 corresponding to "thete'rminal i5ain' thepreceding" figures ofth'e drawings. The other terminal of the diode- :aais connected to the'source 3'5.

of'+B potential through a suitable potentiometer" comprising" theresistor I! and the adjustable tapiZ. Thefotherterininal'of the gasdiodei211 is'groun'ded through a suitable resistor i3, The

potential ofthe' +B source'35 mustbesufliciently large tofire bothdiodes lid and 52b alternately. Theiterminal Tilis connected tolthe sinewave in source Ii through the current limiting resistor is and theblocking, capacitor it. The terminal lilis also connected to groundthrough serially arranged blocking capacitor 'i't'fandresistorsliiandfl'ir The resistors lfiland H are illustratedasliaving a co'mm'on'terminal 38 which provides a' source of potential of complex wave forrn'containing substantially only odd harmonics. Byvirtueof'tliec'apac'itors Hi and lfi' the common.

terminal HTo'fthe gas diodesliz'a and?! 2b main- ,v talifed atanzaverage direct current petentiai between ground and the positivepotential 'ofthe source" 35 whereupon a very. symmetrical wave form isobtained.

Iii .Fig, 6 ofithe drawingsthere is illustrated by the curve Ea thecomplex wave form obtain-- ablewhich 1 contains substantially only oddharmonics. The oscillator sine wave voltage Eeis also. shown.

The potenti al obtainable at-theterminal18 m maybe' supplied th'rough asuitabie switch 8Q actuated by a key 86 .pivotally meunted at 82:

Switch 85 illustrated. as being engageable' with a normally opencontaetfit connected by means: of afsuit'able conductor 3% to suitablefilters and g subsequently tothe: audio amplifier. With the arrangementdisclosed-in Fig. 5 of the'drawings a-very simple way --of obtaining apure-sinewave; ,1- a wave rich in all harmonics, and a Wave richin'oddharmonics only is provided. The three diff-- ferent wave forms areillustratedschematically in "Fig? Sadjacent' the conductor wherein.

particular wave form appears.

ltshouid "be understood that instead of gas... diodes IZ'aand [2bordinary clio'desmight equally] well be employed-and inFig. 7"o'f' thedrawings" spending parts o'fF'ig; 5 are designated by the If, how

the

the identical circuit of Fig. with the tubes 52a and I222 replaced byordinary diodes designated by the reference numerals iii) and 9!respectively, is shown. As illustrated the anode of the diode 9B isconnected to the terminal it) while the oathode of the diode ii isconnected to the terminal 10. It will be understood that when thepotentiometer comprising the resistor H and the variable tap 12 isemployed the odd harmonic output can readily be controlled merely bychanging the common bias. With this arrangement it is possible to tiekeys 62 and 8t together or in other words eliminate one of the keysthereby making possible a keying system employing a minimum number ofcantacts and yet retaining a degree of quality control;

In view of the detailed description included above the operation of themultiple-tone source arrangements described will be apparent to thoseskilled in the art and no further discussion thereof is includedherewith.

While there have been described and illustrated what are at presentconsidered to be the preferred embodiments of the presentinvention, itwill be obvious to those skilled in the art that various changes andmodifications can be made therein without departing from the spirit andscope of the present invention. It is aimed in the appended claims tocover all such changes and modifications as fall withinthe true spiritand scope of the present invention.

What is desired to be secured by Letters Patent of the United States is:

1. In a musical instrument, an oscillation generator for developing anaudible signal having a substantially sinusoidal wave form, means fordeveloping a signal of adiustably complex wave form comprising adistorting circuit including a gas diode characterized by having afiring potential which is substantially higher than the potential forsustaining current flow through said d ode, means for impressing theoutput of said oscillation generator across said diode, the variation inpotential of the output of said oscillation generator exceeding thedifference in Voltage across said diode for firing said diode and forsusta ning current flow therethrough, and means for applying a directcurrent bias across said diode, the value of said bias determining thewave form of said developed signal.

2. In a musical instrument, an oscillation generator for developing anaudible signal having a substant ally sinusoidal wave form, means fordeveloping a signal of adiustably complex wave form comprising adistorting circuit including a gas diode characterized by having afiring potential which is substantially higher than the po tent al forsustaining current flow through said diode, means for impressing theoutput of said oscillation generator across said diode, the variation inpotential of the output of said oscillation generator exceeding thedifierence in voltage across said diode for firing said diode and forsustaining current flow therethrough, means for applying a directcurrent bias across said diode, the value of said bias determining thewave form of said developed signal, and means for obtaining a potentialof said determined complex wave form from across said diode.

3. In a musical instrument, an oscillation generator for developing anaudible signal having a substantially sinusoidal wave form, means fordeveloping a signal of adjustably complex wave form comprising adistorting circuit including a gas diode characterized by having afiring potential which is substantially higher than the potential forsustaining current flow through said diode, means for impressing theoutput of said oscillation generator across said diode, the variation inpotential of the output of said oscillation generator exceeding thedifference in voltage across said diode for firing said diode and forsustaining current flow therethrough, and for applying an adjustabledirect current bias across said diode, the value of said biasdetermining the waveform of said developed signal, and means comprisinga serially arranged capacitor and resistor connected across said diodewhereby a potential of said determined complex wave form may be obtainedacross said resistor.

i. In adevice for producing musical tones including tones of sinusoidalwave form and tones of adjustably complex wave form, the combination ofa source of alternating potential of audio frequency having asubstantially sinusoidal wave form; means for distorting said potentialof audio frequency comprising an electron discharge valve of the typerequiring a substantially greater potential across its terminals toinitiate ionization than to maintain ionization once such ionization hasoccurred, the sinusoidal wave form output of said source having avariation in potential exceeding the difference in potential across saidvalve between that existing at the instant of ionization and that formaintaining ionization; means for connecting said valve across saidsource; and means for impressing a direct current potential across saidvalve of suflicient magnitude so that the sum of said audio frequencypotential of sinusoidal wave form and said direct current potentialequals the ionization potential of said valve at some point during eachcycle of said audio frequency potential whereby the potential acrosssaid valve comprises an audio frequency potential rich in harmonics, themagnitude of said direct current potential determining the wave form ofsaid developed audio frequency potential.

5. In a device for producing musical tones including tones of sinusoidalwave form and tones of adjustably complex wave form, the combina-- tionof a source of alternating potential of audio frequency having asubstantially sinusoidal wave form; means for distorting said potentialof audio frequency comprising a gas diode of the type requiring asubstantially greater potential across its terminals to initiateionization than to maintain ionization once such ionization hasoccurred, the sinusoidal wave form output of such source having avariation in potential exceeding the difference in potential across saiddiode between that existing at the instant of ionization and that formaintaining ionization; means for connecting said diode across saidsource; and means for impressing a direct current potential across saiddiode of sufficient mag nitude so that the sum of said audio frequencypotential of sinusoidal wave form said direct current potential equalsthe ionization potential of said valve at some point during each cycleof said audio frequency potential whereby the potential across saidvalve comprises an audio frequency potential rich in harmonics, themagnitude of said direct current potential determining the wave form ofsaid developed audio frequency potential; and a circuit associated withsaid diode for providing a. means of extracting a potential having awave form rich in harmonics.

6. In a musical instrument, a resistance-caamoares pacitance oscillatorcapable of producing a potential of sinusoidalwave form comprising an-eleetron discharge valve including at least three ---e'lec'trodes oneof which is a control electrode, --a-=parallel-T network connectedacross*two of saidelectrodes, a resistor -connected between said "controlelectrode and ground, means'for producing a frequency vibrato comprisinga low "frequency low impedance sourceconnected inseries -w-ith said--resistor, and a distorting circuit -compr-'ising'a gas diode--connected 'tosaid oscillator for" providing a source of'poten'tialo'f" audiofrequency and of --complex wave form.

" '7." Ina musical instrument, an oscillation gen- "orator-fordeveloping a firstaudible signalhav- *inga substantially sinusoidal waveform, means "for developing two additional signals of complex 'waveform'one containing %both odd and even harmonics and the othercontaining. substantially only odd harmonics, comprising 'adistorting-ma 'cuit including a pair-o'fgasdiodeseaeh character-- izedby having a Thing potential which issubs'tantially-higher'than-thepotential for sustaining "current fiowtherethrough, means for connecting one terminal oi'each-o'fsaiddiodesto-a common terminal, means for-connec'ting the other terminal ofone of said diodes to a sourceof direct current potential, means forconnecting' the other "terminal of said other diode to ground, means"for" connecting said oscillation generator across said other diode, thevariation impotentialof the output '0f 'S&id oscillation generatorexceeding 'the dilference in voltage :at said common term'ina l"required for firing each'of said diodea aoircuit connected to saidcomm-on terininalior supplying 5 :apotential of complex wave "formcontaining-suite stantially only odd harmonies, and means con- 'nectedto said otherterminalof said otherdiode for supplying a potential o'i?complex waveform rich in both odd and even harmonics.

' 8. 'In a musical 'instrument'an oscillation-generator for developing a"first audible s'ignal' ha ving a substantially sinusoidal:wave'=form,wimeans for' developing two-additional signals of complexwave form one containing both odd and even harmonics and the othercontaining only odd harmonics comprising a distorting circuitinc'lud-.ingia pair of 'diodeseach characterized by having :a firing potentialwhich is substantially higher than the potential for sustaining currentflow therethrough, means for connecting thezanode of :one "diOd :and thecathode of the other -.-diode to a common terminal, means forconnecting-the cathode ofsaid one diode to a source of adjustable direct"current potential, means gior connecting the anode of said other diodeto ground, means for connecting :said oscillation generator-across".said otherdiode, the variationi'in" potential. of ithe output of saidoscillation generator exceeding the :difierence 61in -voltage at "saidcommon terminal xrequired"iorifiringneach of said diodea a circuit..:connecte,drto :said common terminal i or "supply-- ing'ra potential.of complex wave form containing substantially only odd harmonics, ,ezndcmeans connected to the anode :ofzsaid other diodefor supplying 'a:potential of comrilexwave "ai orm irioh in both :odd-andeven harmonics.

MER-WIN J.

*REFERENGES GIT-ED The followingreferences are of record tin tiie ifileof :patent:

UNITED STATES PATENTS

